Bought the 24-105 instead of the 24-70 2.8 as need the longer reach for exterior work, plus the IS, but I digress...,

The 50 1.4 is always in the bag, I call it the life saver, don't need it often but great to know it's there. I've lent a friends 1.8 and for me I've found that neither are that good at 1.8 or below, however the 1.4 is clearly better than the 1.8 at 2.0. They are so little and small.

When do I use it?

people shots at functions in low light (which is almost always the situation), focal length not too long (even on a 1.6 crop) anyway less than 35 starts to distort faces - no need for flash and is less obtrusive - which leads to better cameos. Typically use ISO 400 F2.0 and about 1/60th or faster depending on the light

AF clearly better and faster with this lens, and I can see better too!

The 24-105 is still the first choice , though I need to use it at f4 for reasonable bokeh.

if flash fails or forgot the spare batteries :o , or not allowed to use flash - even the 2.8 L's or the 17-55 then need to be wide open - the 50 can get me the shot

when I want the DOF really shallow - however apart from softness, I find under at 1.8 or lower the DOF is so shallow it makes it too tricky in crowds

The 35 1.4 L would work too, but at about $1150 vs $350, it suddenly isn't so attactive

BTW, I don't have the EFS 17-55 2.8 IS, although I often borrow one - IMO whilst big$ it is the best people/party lens on the block - shame the 24-70 doesn't have IS